1. Field of the Invention
The present invention is directed to an integrated optical modulator for modulating optical signals in dielectric wave guides which are disposed on a substrate and with at least one of the wave guides consisting of electro-optical material whose propagation constant is changeable by an application of an electrical potential to electrodes disposed on the substrate.
2. Prior Art
Integrated optical modulators which have a pair of wave guides disposed on a surface of the substrate with at least one of the wave guides being of an electro-optical material whose propagation constant is varied by the application of electrical field produced by applying electrical voltages to electrodes on the substrate have been suggested. In my copending U.S. patent application entitled "An Integrated Optical Modulator", Ser. No. 573,553, filed May 1, 1975 (corresponding to German Application No. P 24 21 285.9, filed May 2, 1974), I disclosed an electro-optical amplitude modulator having a pair of parallel extending wave guides which were positioned either above or below a wide third coupling wave guide and separated therefrom by a thin, low loss dielectric film having an index of refraction n.sub.3 which was less than the index of refraction n.sub.2, n.sub.4, of the two parallel extending wave guides. Since the thickness of the film can be accurately controlled, it was possible to maintain the spacing between the two wave guides and therefore keep the tolerance of the coupling strength in a very narrow range.
This arrangement also offers the advantage that when the two parallel wave guides were disposed beneath the wide coupling or third wave guide, they could be produced in one operation from the same material which enabled the achievement of a good phase matching between the two wave guides. While the value obtained by the equation ##EQU1## (WHERE .beta..sub.O and .beta..sub.u are propagation constants of the upper wave guide and lower wave guides, which lower wave guides are assumed to be identical, and c is the coupling strength) is still involved in the transmission from the first wave guide to the second wave guide, the theory of the three coupled wave guides shows that the reduction in the transmission with the increasing of ##EQU2## IS CONSIDERABLY SLOWER THAN FOR TWO COUPLED WAVE GUIDES. Therefore, it is considerably easier to adhere to the required tolerances for the modulator with the improved structure.
A further possibility for an optical modulator consists in carrying out the coupling and the electro-optical phase modulations both spatially and consecutively in the two coupled wave guides.